A communications network carries information among a number of locations. It consists of nodes connected by links carrying information between them. Rather than have a separate set of wires or separate radio channel for each voice or data conversation, time-division multiplexing (TDM) may be used. For example, ITU-T G.704 consists of 32 channels of 64 kbps, making up 2.048 Mbps. To meet increasing demand for bandwidth, ITU-T G.702 describes the pleisiochronous digital hierarchy (PDH), in which the basic G.704 links are joined together using bit-stuffing to synchronise them. The basic 2.048 Mbps link is known as “E1”, and the hierarchy is based on multiples of four E1, for example, E2 is made up of four E1s and offers 8 Mbps bandwidth. Since E1 is a synchronous TDM link, a channel that has been set up between two users is dedicated until the connection is torn down. Also, dropping and inserting traffic into a PDH link requires a full set of demultiplexers to separate the data stream into individual E1s. Synchronous Digital Hierarchy (SDH) has capability for bandwidth on demand, and is made up of multiples of E1. Both PDH and SDH are circuit-based digital networks, and are referred to as synchronous transfer mode (STM).
Growth in the use of computers has led to the development of packet-based networks. In a packet-based network, the stream of data is split up into packets at the entrance to the network, and is re-assembled at the destination. A packet-based call does not require a dedicated circuit through the network, so allowing packets from one call to be inserted in-between packets from many other calls. Thus packet-based networks can utilise network bandwidth more efficiently than circuit-based networks, and so are better suited to carrying bursty data traffic. Asynchronous transfer mode (ATM) uses fixed-length packets (ATM cells) which allows switching of packets in hardware, resulting in higher speeds than is possible with Ethernet. ATM is very flexible, enabling transmission of different media types such as voice, video and data. It allows dedicated circuits to be set up simultaneously with different bandwidths and Quality of Service (QoS), with high priority and low priority traffic. ATM works in connection-oriented mode, so guaranteeing correct cell-sequencing for packets in a given connection. There are five classes of QoS, Class 0 (always guaranteed), Class 1 (CES and constant bit rate), Class 2 (variable bit rate audio and video), Class 3 (connection-oriented frame relay) and Class 4 (connectionless data transfer such as IP and SMDS).
However, packet-based networks usually do not work well for time-critical applications such as voice, because the packets may experience delay variations whilst travelling through the network. As a result, packets are not received at a constant bit rate, and this has a significant impact on the quality of time-critical connections, such as a telephone call. To allow network operators to carry different applications over a single network, a solution is needed which provides the advantages of both a circuit-based constant bit rate service, and a packet-based, high bandwidth utilisation service.
One approach offered by the ATM Forum, is circuit emulation service (CES) over ATM, described in; “Circuit Emulation Service Interoperability Specification”, Version 2.0, January 1997, AF-VTOA0078.000, available for download from http://www.atmforum.com, the contents of which are hereby incorporated by reference. CES over ATM establishes a logical path through the ATM network. In this respect, CES over ATM is similar to time-division multiplexing (TDM) in that all data follows the same path from one point to another in the network, and so packets should be received in the correct order. An ATM path can accommodate multiple circuits. Depending on the data rate needed for a given circuit, different bit rates can be assigned to different circuits using the same path, so providing a variety of service levels to different users, and allowing greater control over delay variations.
The ATM Adaptation Layer (AAL) converts data from higher-level formats such as X.25, Ethernet, and STM into ATM cells and back again. Within AAL, the convergence sublayer (CS) and segmentation and reassembly sublayer (SAR) take care of applications that require constant bit rate (CBR) and variable bit rate (VBR). ATM Adaptation Layer AAL 1 is used by CES, which requires a very low cell transfer delay and delay variation to carry STM services (for example, streams of E1) over ATM. AAL 1 handles synchronisation, delay jitter, cell loss and wrong cell insertion. The CS and SAR are included in the adaptation layer header, which sits between the ATM cell header and payload data in the ATM cell. When carrying an E1 stream over ATM each E1 frame is smaller than an ATM cell, which means that in order to make full use of the ATM cell payload, E1 frames are subdivided across two ATM cells by the AAL 1 adaptation layer. All ATM cells have a 1-byte AAL 1 header after the ATM cell header, consisting of a 3-bit sequence number, 4-bit cyclic redundancy check (CRC) and 1-bit convergence sublayer indicator (CSI). The CSI bit in odd-numbered cells is used for synchronous residual time stamp (SRTS) synchronisation. Even-numbered ATM cells may also have a structured data transfer (SDT) pointer after the AAL 1 header. The CSI bit in even-numbered ATM cells is set to indicate the presence of an SDT pointer. The SDT pointer points to the first boundary of an E1 frame within the ATM payload, and acts as a check when re-assembling E1 frames. The SDT pointer contains a 7-bit offset having values 0-93 (byte offset split over two ATM cells) or 127 (dummy value), and a parity bit. ATM cells are collected together into a frame of eight cells, and only one SDT pointer is allowed to be active per frame, which makes the system slow to switch channels on and off.
Since E1 is a circuit-based service, it must continue to send data (even if the circuit is idle), until the connection is torn down. When carrying E1 over ATM, however, it is possible to save transmission bandwidth by leaving out any idle channels of the E1 stream. Information about which channels are active at any particular time, may be obtained from the signalling channel, or else by monitoring the E1 channels individually.
ATM Forum specification “(DBCES) Dynamic Bandwidth Utilization in 64 KBPS Time Slot Trunking Over ATM—Using CES”, July 1997, AF-VTOA0085.000, available for download from http://www.atmforum.com, the contents of which are hereby incorporated by reference and also UK patent GB 2276518A, “Statistical gain using ATM signalling” describe a method of saving bandwidth when carrying STM over ATM. This method uses an ATM Adaptation Layer 1 (AAL 1) format that additionally includes a map indicating which STM channels are idle, the idle channels not being transmitted in the ATM cells, and this is achieved using a busy-map for each set of eight ATM cells. The busy-map is a field of length 1-byte up to 4 bytes, which has a 1-bit marker for each of the transmitted timeslots of the E1 bearer (up to 31 timeslots). The marker bit is set if the corresponding timeslot is busy. The length of the frame can be calculated by adding up the number of set bits in the busy-map, and the busy and idle bytes can be sorted using the pattern of bits in the busy-map. This method uses 4 octets (bytes) per frame, which is an overhead. If a cell containing the busy-map is lost, then the following cell may have to be discarded. The damage will extend from the lost cell through to the frame beginning after the next busy-map.
Drawbacks of the busy-map system are as follows:                (a) For data with rapid fluctuations from busy to idle, the busy-map is slow-reacting because it can only be updated at most once per ATM frame (that is eight ATM cells, or a minimum of twelve E1 frames). It is inefficient because it cannot change configuration quickly enough to delete E1 channels that become idle in-between busy-maps.        (b) A more serious problem is that if a channel changes from idle to busy, then there is no way of transmitting the data for that channel until the next busy-map, so resulting in lost data. This is less critical for voice trunking, but limits use for other data applications. The delay in switching on channels is at least twelve E1 frames (12×125 microseconds), equal to 1.5 milliseconds. Thus the busy-map bandwidth utilisation system is lossy, and this can only be overcome by inserting a prohibitively large number of busy-maps.        
Therefore, an improved method is desirable for carrying TDM traffic (such as E1) over ATM, which is both more efficient and less lossy.